Last year's summary reported that cytolytic mouse T cells were successfully retargeted with bispecific antibodies, so that they lysed in vitro, syngeneic breast cancer cells induced by the mammary tumor virus. The bispecific antibody reacted with a triggering site on the surface of T cells a CD3 component within the T-cell receptor complex, and with a virus-related antigen expressed on the surface of the tumor cells. Two requirements for the antitumor activity were that l) the T cells had to be preactivated, and 2) the antibodies comprising the bispecific reagent had to be cross linked and not simply mixed together. We preactivated the T cells by culturing splenocytes from normal donors with irradiated allogeneic cells, bacterial lipopolysaccharide, and recombinant IL-2. Preactivated mouse spleen cells also blocked the growth of syngeneic breast cancer cells in tumor neutralization (Winn) assays in vivo, if retargeted against the tumor cells with the bispecific antibodies. Because the project was closed out this year, we focused our effort on confirming selected observations described above.